The present invention relates to a color cathode ray tube and, more particularly, to stabilization of the potential of the neck inner wall in a cathode ray tube.
Generally, a color cathode ray tube comprises an envelope including a panel, a funnel, and a neck, an inner conductive film formed by adhesion from the inner wall of the funnel to the inner wall of the neck, and an electron gun arranged inside the neck and having a cathode provided in an end portion of the neck and a plurality of grids sequentially arranged from the cathode.
Usually, the electron gun focuses three parallel electron beams, one in the center and two on both sides, onto a fluorescent or phosphor screen and at the same time converges these electron beams. However, the convergence states of the three electron beams change with time due to the influence of a change in the inner wall potential of the neck. As a consequence, a problem such as color misregistration occurs. This problem is ascribed to penetration of the charged potential at the neck inner wall into the main lens of the electron gun, which influences the electric field and changes the trajectories of the two side electron beams. More specifically, the potential of the neck inner wall immediately after an anode voltage is applied reaches a certain fixed potential distribution state under the influence of, e.g., the inner conductive film or a convergence electrode of the electron gun. However, straying electrons generated in the neck collide against the charged neck inner wall to cause secondary electron emission from the neck and thereby gradually raise the neck potential. Consequently, the trajectories of the two side electron beams change to change their convergence states with time. This causes a so-called convergence drift and results in color misregistration.
To solve this problem, Jpn. Pat. Appln. KOKAI Publication No. 53-10959 has disclosed a proposal by which charging by secondary electrons is prevented by setting the surface specific resistance of the inner surface of neck glass to 10.sup.10 to 10.sup.14 .OMEGA./m.sup.2. The use of soda-lime glass having a composition of SiO.sub.2, Na.sub.2 O, K.sub.2 O, CaO, or MgO is described as an example of a resistance film.
Also, Jpn. Pat. Appln. KOKAI Publication No. 64-12449 has disclosed a proposal by which charging by secondary electrons is prevented by forming an insulating coating, having a surface resistance of 10.sup.12 to 10.sup.14 .OMEGA./.quadrature. and a secondary electron emission ratio smaller than unity, on the inner surface of neck glass. Cr.sub.2 O.sub.3 is exemplified as this insulating coating.
In addition, Jpn. Pat. Appln. KOKAI Publication No. 5-205660 has disclosed a proposal by which charging by secondary electrons is prevented by forming a glass enamel layer, containing particles of a substance whose secondary electron emission coefficient is smaller than unity and having a surface resistance of approximately 10.sup.10 to 10.sup.14 .OMEGA./.quadrature., on the inner surface of neck glass. This publication describes that the enamel layer contains Cr.sub.2 O.sub.3 particles.
Unfortunately, it is found by the experiments conducted by the present inventors that even if a high-resistance film such as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 64-12449 or 5-205660 is formed on the inner wall of a neck, it is difficult to completely suppress a convergence drift, and discharge occurs in the tube to significantly lower the reliability of the tube.
For example, a convergence drift occurs when the neck temperature rises while the tube is in operation, and the resistance of the high-resistance film lowers with the increasing temperature to thereby raise the neck potential. Also, the reliability lowers when electrodes constituting the electrode gun in the tube emit electrons by an electric field due to the rise in the neck potential, thereby causing discharge in the tube. The convergence drift and the reliability when a high-resistance film is formed on the inner wall of a neck will be described in detail in order below.
FIG. 1 is a graph showing changes in the neck outer wall temperature with time when a 15" color display tube having a neck outer diameter of 22.5 mm (inner diameter=about 19.5 mm) is in operation. The portion subjected to temperature measurement is the outer wall of a neck where a main electron lens of an electron gun exists. The horizontal deflection frequency was 57 kHz, and the outside air temperature about 25.degree. C. As is apparent from FIG. 1, the neck outer wall temperature starts sharply rising immediately after the operation is started and reaches about 35.degree. C. in twenty minutes. After thirty minutes elapse, the neck outer wall temperature shows saturation and rises to about 45.degree. C.
The inner wall of a neck having an outer diameter of 22.5 mm (inner diameter=about 19.5 mm) was coated with a Cr.sub.2 O.sub.3 film about 15 mm in length along the axial direction of a tube. FIG. 2 is a graph showing the result of measurement of the resistance-temperature characteristic of the film in a vacuum of 10.sup.-3 Torr or less.
As can be seen from this characteristic, the resistance of the film decreases with the increasing temperature. FIG. 2 shows that the temperature dependence of resistance is about d(logR(T))/dT=-0.035 assuming the resistance at T.degree.C. is R(T), and the surface resistance decreases to about 3/10 when the temperature changes from 25.degree. C. to 40.degree. C.
The inner wall of a neck having an outer diameter of 22.5 mm (inner diameter=about 19.5 mm) was coated with the above-mentioned Cr.sub.2 O.sub.3 film about 15 mm in length along the axial direction of a tube. A curve 701 in FIG. 3 represents the convergence drift characteristic of the tube. On the curve 701, the convergence is plotted on the ordinate against time plotted on the abscissa. The Cr.sub.2 O.sub.3 film was formed with a length of about 15 mm along the tube axial direction so as to cover the gaps between grids forming a main focusing electron lens of an electron gun. To minimize charging of the neck by secondary electrons, the convergence measurement was performed by a cross hatch pattern by setting the total beam current of three electron beams to 5 .mu.. After sixty minutes elapsed from the start of the operation of the tube, a change in the convergence was measured by supplying a total electron beam current of 450 .mu.A in a non-measurement state, in order to measure the influence of the neck charging by secondary electrons. Note that the positive convergence direction corresponds to under convergence and the negative convergence direction over convergence. The outside air temperature during the measurement was about 25.degree. C.
Also, a curve 702 in FIG. 3 represents a change in the electrical resistance with time of the Cr.sub.2 O.sub.3 film described above. This is calculated from the change in the neck temperature with time shown in FIG. 1 and the temperature characteristic of the electrical resistance of the Cr.sub.2 O.sub.3 film shown in FIG. 2. On the curve 702, the film resistance is plotted on the ordinate against time plotted on the abscissa.
As can be seen from the convergence drift characteristic indicated by the curve 701, the convergence immediately after the tube starts operating is an over convergence of about 0.3 mm. The convergence quickly decreases during the time interval from fifteen to twenty minutes and converges to almost zero after thirty minutes elapse. This indicates that the neck potential of the tube is charged to a comparatively low voltage immediately after the start of the operation and changes to a relatively high potential and stabilizes with passing of time. After sixty minutes elapse, the convergence remains unchanged even when a high beam current is supplied. That is, the neck potential is not changed by secondary electrons, indicating the effect of a high-resistance Cr.sub.2 O.sub.3 film.
The convergence drift characteristic indicated by the curve 701 and the electrical resistance of the Cr.sub.2 O.sub.3 film indicated by the curve 702 change with time in substantially synchronism with each This supports the fact that the convergence drift immediately after the tube starts operating is accounted for by a change in the electrical resistance of an antistatic film with time.
Compared to the resistance of an antistatic film at low temperatures, the resistance at high temperatures after fifteen to twenty minutes elapse sharply drops to about 3/10. Consequently, the film potential lowers to make the convergence drift in the direction of under convergence.
In the prior arts disclosed in Jpn. Pat. Appln. KOKAI Publication Nos. 64-12449 and 5-205660 as described above, charging of the neck potential by secondary electrons can be prevented. However, if a film having a large resistance-temperature characteristic such as a Cr.sub.2 O.sub.3 film is used, the problem of a convergence drift newly arises due to heat generated by the tube. This convergence drift cannot be completely prevented. Also, the electrical conduction of soda-lime glass disclosed in Jpn. Pat. Appln. KOKAI Publication No. 53-10959 is ion conduction. Therefore, the temperature dependence of resistance is large, and this poses the problem of a convergence drift due to heat generation by the tube as in the case of Cr.sub.2 O.sub.3.
In addition to the above problems, another serious problem arises when the inner wall of the neck is coated with a high-resistance film with a large resistance-temperature characteristic.
In the manufacturing process of a cathode ray tube, so-called spot knocking processing is generally performed by forcedly generating an intratube spark by applying a high voltage to the assembled cathode ray tube, in order to improve the withstand voltage characteristic of the cathode ray tube. However, even when a high voltage is applied to the tube in which the inner wall of the neck is coated with a high-resistance film having a resistance-temperature characteristic, a spark or a leakage current not resulting in a spark is generated, so the spot knocking processing cannot be well performed. Consequently, the obtained cathode ray tube has no satisfactory reliability.
FIG. 4 is a graph showing the relationship between the neck temperature and the leakage current from a focusing electrode in a conventional tube in which a Cr.sub.2 O.sub.3 film is formed as a high-resistance film on the inner wall of the neck. As shown in FIG. 4, the leakage current abruptly increases when the neck temperature exceeds about 65.degree. C. The reason for this is considered as follows. That is, as the neck temperature rises, the electrical resistance of the high-resistance film lowers, and the neck potential rises accordingly. This increases the intensity of the electric field concentrated from the neck inner wall to the focusing electrode. As a consequence, the field emission current of electrons from the electrode increases.
The abrupt increase in the leakage current at high temperatures is probably due to failure in maintaining the withstand voltage characteristic of the tube at high neck potentials. More specifically, the neck temperature during spot knocking is lower than the maximum neck temperature when the tube is in operation, although the neck temperature also depends upon the outside air temperature and some other conditions. For example, when spot knocking processing is performed at 25.degree. C., the resistance of the high-resistance film is approximately 2.times.10.sup.13 .OMEGA. as shown in FIG. 2. On the other hand, the neck temperature rises to about 65.degree. C. when the tube is in operation, although it also depends upon other conditions. At that time, the resistance of the high-resistance film is approximately 1.4.times.10.sup.12 .OMEGA. as shown in FIG. 2; the resistance of the high-resistance film lowers by about 93%. As described above, although the spot knocking processing is performed with a high film resistance, the film resistance lowers while the tube is operating, and this increases the neck inner wall potential. Presumably, when the neck potential rises, the withstand voltage characteristic of the tube cannot be maintained any longer, and this generates a spark or a leakage current not resulting in a spark when the tube is in operation.
While the tube is operating, the neck temperature rises to about 45.degree. C. when the outside air temperature is 25.degree. C. If the outside air temperature is high or heat radiation around the tube is insufficient, the neck temperature can rise to 65.degree. C. or higher. For example, when the leakage current exceeds 0.3 .mu.A, the focusing characteristic of the tube obviously deteriorates. Even if a leakage current smaller than that value flows, discharge occurs in the tube, and this can destroy electrical circuitry which supplies a voltage or the like to the tube. Furthermore, the reliability of the tube significantly suffers.
According to the experiments conducted by the present inventors as described above, even when neck charging by secondary electrons is prevented by a high-resistance film such as disclosed in the prior arts, if the film resistance of this high-resistance film changes with temperature, a cathode ray tube generates heat to bring about a convergence drift. Therefore, these prior arts cannot completely suppress the convergence drift.
Additionally, a high-resistance film as disclosed in the prior arts has a large temperature coefficient of resistance. Therefore, if the neck temperature rises when the tube is in operation, a leakage current flows. This significantly decreases the reliability of the cathode ray tube.